After a multitude of tests, Cooper was diagnosed with a rare condition called Catecholaminergic Polymorphic Ventricular Tachycardia, or CPTV. It is a disorder characterised by an abnormal heart rhythm and, when coupled with too much adrenaline, can send the heart into cardiac arrest.

It is thought to be the cause of up to 15 per cent of all unexplained cardiac deaths in young people. Cooper had never shown any symptoms before.

"When I saw him sitting up in the hospital the next week, I was really relieved," Ms MacCaskell said.

Nine-year-old Cooper Appleyard has been diagnosed with a rare heart condition.

Cooper's parents are incredibly grateful that Ms MacCaskell knew CPR. They are in no doubt she saved their son's life.

"If he did not have that CPR, he would have died. They doctors have said that," Mr Appleyard said.

"How do you thank someone for saving a child's life? I'm just so grateful Danielle was there," Mrs Ewan-Appleby said.

"I think it's an important skill to have, everyone should be encouraged to learn CPR," Ms MacCaskell said.

Cooper will have a defibrillator fitted into his chest. If his heart goes into cardiac arrest again, it will deliver an electrical shock to save him.

Stuart Goddard has a defibrillator permanently fitted to his heart to keep it pumping. Picture: Jake Nowakowski

AFL player Stuart Goddard has the same device fitted in his chest and Belgian soccer player Anthony Van Loo has the same condition and he made world news when he went into cardiac arrest midgame and his ICD restarted his heart.